(1) Field of the Invention
This invention generally relates to a differential calibration signal generator for an amplifier. More specifically this invention is directed to a calibration signal generator that is adapted to be implemented in integrated circuit form.
(2) Description of the Prior Art
A variety of sensors, typically piezoelectric sensors, require high-impedance, low-noise interface circuits with amplification and other signal conditioning functions. Sonar hydrophones, piezoelectric accelerometers and pressure sensors are all examples of piezoelectric sensors characterized by a very small capacitance and thus high impedance. In essence such a piezoelectric sensor can be considered a voltage source in series with a capacitor. One application for such sensors is a high frequency beamforming acoustic array. Such an array requires very small, omni-directional acoustic sensors and miniature accelerometers which require small mass.
Initially preamplifiers for such transducers were built from discrete electrical components and often times were larger and more massive than the sensor itself. U.S. Letters Pat. No. 4,013,992 to Dewberry et al. discloses one such preamplifier. It was found, however, that sensor performance could be degraded due to signal interaction particularly if the preamplifier were proximate the sensor. The signal interaction problems could, of course, be overcome by relocating the preamplifier a significant distance from the sensor. However, capacitance in a resulting interconnecting cable could attenuate the sensor signal and thus reduce the sensor signal-to-preamplifier noise ratio.
My U.S. Letters Pat. No. 5,339,285 for a monolithic low noise preamplifier for piezoelectric sensors discloses a single, monolithic integrated circuit that can be mounted directly onto or inside a miniature sensor without degrading the sensor's performance. The preamplifier performs signal amplification with a fully differential amplifier that includes common-mode feedback. A pair of feedback capacitors together with the sensor capacitance control the voltage gain of the preamplifier over much of its useful operating range of 100 Hz to 100 kHz. The preamplifier circuit has feedback resistors that discharge any accumulated dc voltage which might appear on the capacitors. The feedback resistors are also components in a high pass filter that rejects low frequency background noise from the sensor.
Although such a preamplifier is particularly well adapted for implementation as an integrated circuit and is useful in a number of applications, some transducers, particularly hydrophones, can generate a signal having a wide dynamic range. This range oftentimes can generate an amplified signal that exceeds the operating range of a following analog-to-digital converter or the capabilities of other circuits. Consequently it would be desirable to employ an amplifier with variable gain, typically a step-wise variable gain, that would accommodate different portions of a total dynamic range.
Circuits providing step-wise gain generally incorporate a switched resistor network typically in a feedback circuit. For example U.S. Letters Pat. No. 4,354,159 (1982) to Schorr et al. discloses a prescription attenuator having cascaded L-pad sections that are coupled together to form an attenuator network. In this network each cascaded section includes a single-pole switch for activating its respective section. The network impedance values are selected so that the single L-pads section activations produce actual attenuations slightly more than an ideal level of attenuation whereby multiple section activations tend to keep the error evenly distributed about an ideal level of attenuation.
U.S. Letters Pat. No. 5,351,030 (1994) to Kobayashi et al. discloses a variable attenuator for attenuating gain of an analog signal in accordance with a digital signal. This attenuator has a plurality of attenuation resistor units, a plurality of switching units, and a plurality of impedance compensation resistor units. The attenuator resistor units are connected in series for attenuating an input signal, and the switching units are provided for the attenuation resistor units. One of the switching units is selected in accordance with a digital control signal applied from an external source. Each of the impedance compensation resistor units is connected in series with the respective switching units to compensate an output impedance to a specific value regardless of the state of the switching units. Consequently the variable attenuator in this reference can correctly control the level of an analog signal by a digital signal.
U.S. Letters Pat. No. 5,387,879 (1995) to Satoh discloses a gain controllable output buffer amplifier circuit having a reduced circuit area. In this circuit an operational amplifier has a grounded non-inverting input and an inverting input connected through an input resistor to a signal input node. A tapped feedback resistor having a plurality of intermediate taps is connected between an output and the inverting input of the operational amplifier. Each of the intermediate taps is connected through a transistor switch in common to a non-inverting input of an output buffer amplifier, which has its output connected to a signal output node and an inverting input of the output buffer amplifier itself. The transistor switches are controlled by a switch control circuit in such a manner that only one of the transistor switches is selectively turned on.
A number of references disclose different types of circuits for operating with single-ended or differential inputs for producing differential outputs to overcome noise problems and control common mode output voltages. For example, U.S. Letters Pat. No. 3,668,543 (1972) to Bailey discloses a transducer amplifier system; U.S. Letters Pat. No. 4,933,644 (1990) to Fattaruso et al., a common mode feedback bias generator for operational amplifiers. U.S. Letters Pat. No. 5,166,635 (1992) to Shih discloses a digital data line driver that receives differential input signal and generates a differential output signal. U.S. Letters Pat. No. 5,381,112 (1995) to Rybicki et al. discloses a fully differential line driver circuit with common mode feedback; and U.S. Letters Pat. No. 5,428,316 (1995) to Molnar, a power amplifier with quiescent current control that utilizes fully differential amplifiers and a high impedance closed loop common mode feedback control circuit.
Notwithstanding the existence of such fully differential amplifiers and the known disadvantages of operating single-ended amplifiers in applications that require the handling of large common mode voltages, such systems are still in use. This occurs notwithstanding the fact that the operational amplifier stage gain is a function of the input common mode voltage and large common mode voltage swings can increase signal distortion. However, it has been found very difficult to implement a fully differential architecture utilizing step gain functions implemented with resistance ladder networks.
U.S. Letters Pat. No. 4,855,685 (1989) to Hochschild discloses a precision switchable gain circuit that includes an operational amplifier operating in a differential mode. An input leg is comprised of a series resistor and a MOS transistor. A plurality of feedback legs are formed, each comprising one or more resistors that are equal in value to the input resistor and connected in series with a switch transistor. The proportion of the series resistance of the transistor in a given feedback leg to the series resistance of the transistor in the input leg is equal to the proportion of the fixed resistance in the feedback leg and input leg. The value of the series resistance of the feedback transistors therefore factors out the series resistance of the input transistor in the gain calculation resulting in reduced harmonic distortion.
Although the Hochschild patent discloses a fully differential amplifier with a step-wise gain control, the design is dependent on certain criteria that limit the application. For example, the total resistance for any given feedback leg must equal a predetermined multiple of the input resistor. Each feedback leg in most of the Hochschild embodiments is independent. There is no resistance sharing, so that the total resistance of the circuit is large. As known, the physical size of the resistors is proportional to the total resistance so increasing the total resistance increases the integrated circuit size thereby decreasing yield and increasing costs. Increasing the resistor area further increases the parasitic capacitance from the resistor to the substrate that has the effect of reducing circuit performance particularly by reducing the bandwidth over which gain is stable.
FIG. 4 of the Hochschild patent discloses a version in which resistances are shared. Although this will tend to reduce the total resistance of the circuit, the reduction occurs under only specific circumstances that limit the gain steps to integer multiples. Thus, there remains a need for a fully differential amplifier with a step-wise gain control where the gain at each step can be any arbitrary number greater than 1.
In many applications, and particularly hydrophone applications, it is also desirable to incorporate the capability of calibrating the amplifier circuit by measuring the gain of the amplifier circuit. For example, U.S. Letters Pat. No. 4,689,578 (1987) to Spychalski discloses a hydrophone pre-amplifier with self calibration. More particularly, a differential charge amplifier design for a hydrophone preamplifier in accordance with this patent includes a calibration circuit for in situ measurement of the gain of the hydrophone amplifier and pre-amplifier. The hydrophone signal is input to a pair of amplifier stages which form a balanced differential input charge amplifier. The signal from the input charge amplifier is amplified by a second preamplifier stage and then passed to a balanced output cable driver. The calibration circuit is enabled by an external signal to inject a calibration signal into the input charge amplifier front end thereby to enable the output signal to incorporate the calibration signal in a measurable form.
FIGS. 1A and 1B depict another prior art implementation of a hydrophone preamplifier with self calibration. Although shown in detail in FIGS. 1A and 1B, it will be sufficient for the understanding of the operation of this circuit to state that a fully differential input signal appears across input connections 10A and 10B spanned by a limiting circuit 11 to drive a differential amplifier 12. The output of the differential amplifier 12 differentially drives a single-ended operational amplifier 13 generating an output at a first output terminal 14A which is coupled to a feedback circuit including an operational amplifier 15 that generates an output at a second output terminal 14B. Consequently the circuit in FIG. 1A does produce a differential output.
The circuit of FIG. 1A also includes resistors 16A and 16B that constitute calibration injection resistors that convey a fully differential calibration signal to the differential amplifier 12. FIG. 1B depicts a structure for generating the fully differential calibration signal. In this implementation a calibration input, typically a digital input signal from an external source, is applied to a terminal 20 for driving a one-bit digital-to-analog converter 21 supplied by a power supply 22. The resulting output from a common switching point 23 is fed to a precision level controller 24 that drives a two-stage amplifier section comprising operational amplifier 25 and a unit gain inverting buffer operational amplifier 26. An output selector 27 then conveys the signals from the amplifier 25 and unit gain amplifier 26 to output terminals 28A and 28B respectively. The terminals 28A and 28B connect to corresponding terminals 28A and 28B in FIG. 1A for conveying the fully differential signal from the circuit in FIG. 1B to the differential amplifier 12 in FIG. 1A. In this particular implementation, the circuitry in FIGS. 1A and 1B are separate circuits and incorporate a number of components that are not readily adapted for integrated circuit applications.
Notwithstanding the availability of fully differential, step-wise gain controlled operational amplifiers, calibration circuits and the like in the prior art, circuitry such as shown in FIGS. 1A and 1B continues to be utilized. Although the circuitry in FIG. 1A could be substituted in a single gain amplification, there still has been no solution for providing amplification of a signal having a wide dynamic range with minimal distortion. Notwithstanding the disclosure in the Hochschild patent, these and other prior approaches when implemented in an integrated circuit environment, required large values of the resistors with the previously described disadvantages. Consequently, there remains a need for a simplified design with a reduced number of components and a reduced resistance in order to optimize the construction of such an amplifier as an integrated circuit and a need for such a circuit that provides a calibration function.